This protocol will evaluate the utility of the two GFR markers, creatinine and cystatin C, in comparison with measured glomerular filtration rate (GFR) by iohexol clearance in 375 Black and White adult participants of the population-based Multi-ethnic Study of Atherosclerosis (MESA) cohort from the Johns Hopkins clinical center. Although GFR estimation by creatinine has improved over the past decade, there has been inadequate inclusion of persons over 65 years of age, and no study of GFR measurement has occurred within a population-based sample of Blacks and Whites in the United States. To advance the detection and definition of CKD across a broad age range of diverse adults,, we must focus on the higher ranges of GFR, and we must identify the optimal strategy for GFR estimation. The first Aim of this cross-sectional study will be to determine the influence of Black race-ethnicity on serum levels of filtration markers (creatinine and cystatin C) in the general population, and to modify current GFR equations to capture optimally the influence of Black race. The second Aim will be: to evaluate age-related decline in GFR; to compare the relative ability of the filtration markers to capture declining GFR with age; and, to modify current GFR equations to incorporate the influence of age. In executing these Aims, we will determine the optimal biomarker(s) for GFR estimation creatinine, cystatin C, or both. In addition, blood specimens will be stored with the potential for examination of future filtration markers. To accomplish our objectives, we will measure iohexol-GFR in approximately 375 Black and White MESA participants, spanning a target age range of 55-95. This study could be a major contributor to the National Institute of Healths efforts to address the persistent racial disparities in the burden of kidney disease in the United States.